1. Field of the Invention
The present invention relates generally to an electrically heatable type catalyst which is disposed in an exhaust gas passage of an engine. More particularly, the present invention relates to an apparatus for supplying the secondary air to the heatable type catalyst.
2. Description of the Related Art
Japanese Unexamined Patent Publication 1-227815 discloses an exhaust gas purifying appartus. In this apparatus, the secondary air is supplied to a three way catalytic converter disposed in an exhaust gas passage of an engine when the engine was started in the cold state. By this secondary air supply, the reduction of exhaust gas emission discharging to the atmosphere is realized.
In recent years, for the purpose of earlier activation of the catalyst, the catalyst including an electric heater has been developed. For supplying the secondary air to the catalyst, a developed type of electric air pump has been proposed as a replacement of a known mechanical air pump operably connected to the engine. The proposed air pump has an electric motor which is actuated when required. Therefore, the modification of the prior art is proposed to reduce the exhaust gas emission when the engine is started in the cold state. This is realized by an apparatus which actuates the electric heater of the catalyst when the engine is started in the cold state, and operates the air pump for supplying the secondary air to the catalyst.
Japanese Unexamined Patent Publication 5-59940 discloses an example of the apparatus, namely a catalytic purifying apparatus for an internal combustion engine. In this apparatus, when the temperature of the catalyst is raised to the operational point by actuating the electric heater when the engine is started in the cold state, the electric air pump is activated. In this apparatus, even when the secondary air is supplied to the catalyst of which the temperature has not been raised up to the operational point, heat loss of the catalyst caused by the air flow is greater than of heat generated by the oxygenate reaction. Therefore, before the temperature of the catalyst is raised up to the operational point, the catalyst is heated up by means of the heater so as to become active earlier, without supplying the secondary air to the catalyst.
However, according to the above modified example of the first prior art, the electric heater and electric air pump are simultaneously actuated as the engine is started in the cold state. In this state, the secondary air takes away the heat of the catalyst the temperature of which has not been raised up to the operational point during the heating-up operation for the catalyst. Consequently, the period of time required for the catalyst to become active is extended and the purifying ability of the exhaust gas may be reduced. Further, the electrical power may be wasted because the heater is actuated. Furthermore, the high crush current will flow when the electric motor of the air pump is actuated. Therefore, when the electric heater and electric air pump are simultaneously actuated, a heavy load is applied to a power source such as a battery. As a result, the excessive discharge of the battery may shorten the life span thereof. The load demanding to an alternator mostly disposed in the vicinity of the battery is rapidly increased in response to the lowered battery voltage. This causes the engine to engage in the erroneous rotary motion.
According to the second prior art, when both electric heater and electric air pump are continuously operated even after the temperature of the catalyst was raised up to the operational point, the catalyst may be excessively heated up beyond the required degree by applying the electrical power thereto. The electric power may be wasted by continuously actuating the electric heater of the catalyst and the electric air pump.